The present invention relates to a diaphragm control circuit. Recently, there has most often been employed so-called through-the-lens photometry with fully opened diaphragm, in which a light quantity or luminance coming through the fully opened diaphragm from an object to be photographed is received by a photoconductive element disposed within the camera, and thereby a desired photometry is achieved so that a proper diaphragm value for proper exposure is calculated from a luminance measured as coming through the objective with the diaphragm fully opened, a preset shutter speed, sensitivity of the film used and an exposure multiple. After shutter release, the diaphragm is mechanically regulated to such calculated value. With this method of photometry, it is required to detect a diaphragm value at every moment in the course of progressive diaphragm regulation, to feed back this to a calculation circuit of the camera to be compared therein with the calculated diaphragm value, and thereby to arrest said progressive diaphragm regulation upon coincidence of both values so as to fix the diaphragm value attained at this moment.
Such method based upon detection of progressive variation in the movement of the diaphragm may be substantially classified into two types. The first type is such that the light quantity actually coming through the lens diaphragm is measured and the second type is such that a pulse generator carried by part of a member operatively associated with the lens stop down member generates pulses the number of which depends upon the actual extent of diaphragm stopping down. These pulses are counted to detect the actual diaphragm value at every moment. With the former type, it is extremely difficult to detect an accurate diaphragm value for photographing in a relatively dark environment, since the light quantity passing through the objective is sharply reduced as the diaphragm is stopped down near the lowest value. In the latter type, on the other hand, the detection of accurate diaphragm value is possible even in a dark environment, since the detection is independent of such light quantity passing through the objective. The latter is also advantageous in that the precision at which an actual diaphragm value at every moment can be detected may be substantially improved by increasing the unit number of pulses to be generated at every step of stopping down; if there are provided a plurality of steps of stopping down representing the light quantity varying in a series of multiples, a predetermined unit number of pulses is associated with each step of stopping down. However, the requirement for mechanical precision imposes a limitation on the extent to which the unit number of pulses in each step can be effectively increased.